The enzymatic hydrolysis of retinyl esters is a key process in the body's overall vitamin A homeostasis. Although the liver plays a central role in vitamin A metabolism, our understanding of the hepatic enzymes that catalyse the hydrolysis of retinyl esters is incomplete. Only one such enzyme has been isolated in pure form, a neutral, bile salt dependent retinyl ester hydrolase (BSDREH), and this has only been recently achieved. The objective of this proposal is to obtain new and detailed information on the functions, regulation, and physiological role of the retinol ester hydrolases in the liver. This will improve basic understanding of factors controlling vitamin A homeostasis. Such information is important given the critical role of retinoids in the development and maintenance of differentiated tissues and the roles that retinoids may play in preventing or treating certain cancers. In Aim 1 comparisons of the hepatic BSDREH and pancreatic cholesteryl ester hydrolase at the levels of protein structure, CDNA sequence, and enzyme properties will be made in order to test the hypothesis that the hepatic and pancreatic enzymes are the same protein or isoforms. In Aim 2 the regulation of the hepatic BSDREH will be probed by testing the hypothesis that enzyme synthesis is under translational control and that enzyme activity is regulated by postranslational modifications. In Aim 3 the bile salt-independent retinyl ester hydrolases of rat liver will be isolated in pure form for detailed study of their biochemical and enzymological properties. In Aim 4 hepatoma cell culture model systems will be used to test the hypothesis that the bile salt dependent and bile salt-independent retinyl ester hydrolases play physiologically important roles in one or more pathways of hepatic vitamin A metabolism.